For an in vivo assessment of the timing of fiber formation and precursor number in wild type and myostatin deficient mice, we have collected mouse muscle from many stages of development and begun the analysis. We are developing an embryonic stem (ES) cell model of skeletal muscle differentiation using a mouse cell line so that we can alter myostatin signaling during different stages of muscle determination and differentiation. We have determined that different ES cell lines are inherently different in the amount of muscle they can form. In addition, we have determined which culture conditions are best for differentiation for these lines.